Teaching grandmother to suck eggs. Teaching (your) grandmother to suck eggs is an English language saying that refers to a person giving advice to another person in a subject with which the other person is already familiar (and probably more so than the first person).
It is quite an old phrase and is included in John Stevens' translation of Quevedo's Comical Works, 1707: "You would have me teach my Grandame to suck Eggs.".
Teaching grandmother to suck eggs is an English-language saying, meaning that a person is giving advice to someone else about a subject that they already know about (and probably more than the first person).